Some 256Kx1 DRAMs that can be looked up at http://www.datasheets.org.uk/ appear at the end of this post. The trailing -NN access time speed designator should not be included in the part number lookup as its meaning will appear as an explanatory note within the base part number datasheet. The CMOS or NMOS designation is just a technology comment and should likewise not be put into the part number search field.

There are parts faster than the examples shown below. To look up a part datasheet requires knowledge of the part number, which may only be partially indicated on the part marking. The part marking often includes the mfg logo and some trailing digits, omitting the all important prefix letters and numbers. Faced with such a chip, you have to guess the mfg then from the examples that you know, add prefixes to try a search.

Given

4256 try SMJ4256

41256-15 try uPD41256, MSM41256, or KM41256 (one at a time!)

81257-15 try MB81257

41256C try uPD41256, then see if a C datasheet appears as a separate choice.

I leave it to interested readers to verify that page mode compatibility, or whatever, is met or not met by inspecting the datasheet of the parts at hand. The parts listed below are just examples of DRAM part numbers in use by leading manufacturers at that time. It's not a complete list, and most certainly does not imply fitness for any particular use.

100 NS

Hyundai HY53C256L-10 CMOS

Samsung KM41C256-10 CMOS, KM41C257-10 CMOS, KM41C258-10 CMOS

OKI MSM41256A-10 NMOS, MSM41257A-10 NMOS

Fujitsu MB81256-10 CMOS

120 NS

Hitachi HM51258-12 CMOS

Fujitsu MB81256-12 CMOS

NEC uPD41256-12 NMOS, uPD41257-12 NMOS

OKI MSM41256A-12 NMOS, MSM41257A-12 NMOS

Samsung KM41256A-12 NMOS

150 NS

Hitachi HM51258-15 CMOS, HM50256-15 NMOS

NEC uPD41257-15 NMOS

OKI MSM41256A-15 NMOS, MSM41257A-15 NMOS

TI SMJ4256-15 NMOS

Samsung KM41256A-15 NMOS

200 NS

Hitachi HM50256-20 NMOS

TI SMJ4256-20 NMOS